1. Field of the Invention
The present invention relates to a LAN connecting device for connecting LAN (local area network) and WAN (wide area network) such as a public network, etc. In particular, the invention relates to a LAN connecting device provided with a function (hereinafter called "inverse MUX function") for bundling a plurality of public lines (e.g., ISDN private channel) and treating them seemingly as channels of high-speed bands.
2. Description of the Prior Art
Currently, LAN (local area network: intracompany/local information communication network) is popularly used in such limited areas as factories, research institutes, universities, and the like (hereinafter termed "offices" generically) for connecting computers in order to allow mutual transmissions of data, at high speeds. Such a LAN can not only be built independently within an area where computers are bundled together but also be connected to the public networks such as ISDN, etc., allowing communications with outside information equipments and databases via such public networks. Even in the case where a plurality of offices respectively having LAN communication operational environments built up are dispersed on different geographical areas, use of this system allows communications among all these offices as if they were connected via one LAN by connecting the LANs of the respective offices each other through the public networks.
In order to connect such LAN and the public networks, a LAN connecting device called a bridge or a router is used. The bridge is a LAN connecting device having a function for connecting LANs and filtering a packet in accordance with the Data Link address of the forwarded packet. The router is another LAN connecting device provided on the branching point of a plurality of LANs having a function for determining the path of the packet in accordance with the network address of the forwarded packet.
When linking such LAN connecting device to the other LAN connecting device via the public networks, a private line of a fixed band was used as a relay channel. In this case, the ISDN has a band of 64 kbps per one private line. However, as the amount of data to be transmitted increases, congestion will be generated if only one channel is used. For this reason, conventionally the LAN connecting device has been used in combination with a multiplexer function for distributing data to a plurality of parallelly arranged channels to transmit them to the public networks and for multiplexing data sent respectively from the plurality of such parallelly arranged channels to restore the data. Hereinafter a device for arraying data in parallel and sending them to channels is called "a multiplexer". In this way, by virtually bundling a plurality of channels, the bands of the channels are seemingly expanded and the channels can be treated as channels of high-speed bands.
Further, there is a case where the multiplexer is provided with a function for virtually increasing or decreasing the number of channels to be bundled in accordance with the amount of data to be sent, that is, the amount of traffic. In such a case, the multiplexer monitors the utilization rate of already connected channels. Then, when the amount of traffic exceeds a specified threshold level(e.g., 70%), another channel will be added to increase the band. For example, in the case where communications are carried out at the band of 64 kbps utilizing one ISDN private line, when traffic from LAN exceeds 44.8 kbps which is 70% of 64 kbps, another private line is set and the communications will be done at the band of 128 kbps.
However, at such a conventional LAN connecting device, it has been impossible to efficiently obtain channels matching the amount of traffic. Problems relating to the conventional device will be described in detail below.
&lt;Problem 1&gt;
As described above, at the conventional LAN connecting device, the number of channels to be virtually bundled was increased or decreased depending on the amount of traffic. By such a construction the LAN connecting device allows addition of channels to be connected when the amount of data increases during communicating. However, if a communication equipment (a LAN connecting device, etc.) at a call incoming side is engaged in communications with the other communicating equipment using a trunk line (a communication channel connecting a communication equipment and a public network), it may not be possible to connect the number of channels needed by the LAN connecting device at a call originating side. When such a situation arose, the LAN connecting device at the call originating side terminated its call or carried on communications only by already connected channels. Or more often than not it waited for the time when the line of the receiving side became empty by repeating calls at every passages of specified time.
However, in this case the LAN connecting device at the call originating side needs to make a call processing at certain time intervals while executing transmission of LAN data (routing, etc.) For this reason, loads on the CPU (Central Processing Unit) of this LAN connecting device usually increased. In order to reduce such loads on the CPU, it may be possible to equip the LAN connecting device with a multi-CPU system and to let the respective CPUs share data transmission and call processings. By such a configuration, however, the device is disadvantageous in that the costs will increase and control of the processings will become complex.
Further, if the LAN connecting device at the call originating side is constructed in such a way as to simply repeat its calls at every time intervals, even when the trunk line of the communication equipment (LAN connecting device, etc.) at the call incoming side becomes available, the LAN connecting device at the call originating side may not be able to immediately make a call because it is on a standby period set by a timer. When such a case happens, during the lag from the time when the trunk line of the communication equipment at the call incoming side becomes available to the time when the LAN connecting device at the call originating side make a call again, the other communication equipment (LAN connecting device, etc.) may connect to the available trunk line thereof. In other words, even when the LAN connecting device at the call originating side is actually linked with the communication equipment at the call incoming side via the insufficient number of channels and makes calls so as to increase the number of channels, these calls will not have precedence over calls by other communication equipment at the incoming side. Thus, though irrational, depending on timing it happened that the calls later made by the other communication equipment were received while the number of lines needed by the communication equipment at the call originating side to secure sufficient bands were not connected. This problem may not be solved even by adopting the multi-CPU system.
&lt;Problem 2&gt;
When the inverse MUX function is executed by the multiplexer, the band of data transferred and processed within the multiplexer, that is, within the LAN connecting device will be changed in accordance with the increase or the decrease of the number of channels to be connected. For example, when it is connected to the ISDN only by one private channel as shown in the above example, the multiplexer will only need to process data at the band of 64 kbps. However, as data are multiplexed and distributed by the multiplexer when it is connected by two private lines, the multiplexer will have to transfer and process data at the band of 128 kbps which is twice as large as 64 kbps. Thus, as the number of channels increases, the speed for transferring and processing data within the LAN connecting device will have to be increased.
The multiplexer and the other devices, for example, the interface device for the LAN within the LAN connecting device are connected by interfaces specified by CCITT recommended V.35, X.21, etc., and clocks are supplied from the multiplexer side to the other device sides. Thus, when changing bands, clock frequencies must be changed matching the changes of the bands within the multiplexer and supplied to the other devices. Then, at the other devices, data are transferred and processed based on these clock frequencies. However, at the other devices, due to abrupt changes of the clocks it took long time to acquire clock synchronism and data were lost during their changes.